NH4-dot-diagramThe cation exchange capacity (CEC) of the soil is merely a way of measuring the amount of sites in soils or soil humus that have a negative charge. These hold on to positively charged particles (cations) by means of its electrostatic properties. The amount of such exchange sites measures the capacity in the soil to keep nutrients, or the cation exchange capacity (CEC). So a soil’s CEC is the sum of negatively charged nutrient exchange sites  per unit weight or volume. CEC is calculated in milligram equivalents per 100 grams (meq/100g). Adding the concentrations of each cation gives a figure of the overall  CEC. A figure above 10 (meq/100g) is preferred for normal plant growth . However substrates with high levels of humus can have a CEC of 30 (meq/100g) or more.

The 5 most important cations in soils are calcium (Ca++ ), magnesium (Mg++), potassium (K+), sodium (Na+) and aluminium (Al+++).

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Cations and Anions

What we really need to understand is that the greater the CEC value, the more nourishment a substrate or soil can have ready for use. The higher the CEC value, the more effective it can be for improving plant growth, vigor and health. All very small particles, not just humus and clay, carry electrical charges. The part of the nutrient that carries the electrical charge are called ions. Ions with a positive charge are called cations and ions carrying a negatively charged are called anions.

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Humus and Available Nutrients

Depending on the soil or substrate, there are a few ingredients that have cation exchange capacity. The element having the highest CEC would be humus. Organic substrates all contain a good amount of this organic compound. Cations held by the electrostatic force of the soil’s humus can be easily exchanged for other cations within the soil making them readily available for plant uptake in the rhizosphere. Therefore, the CEC is crucial for knowing if there are sufficient amounts of AVAILABLE nutrients in the soil that have a positive charge.

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Common Nutrient Cations And Anions

Some important positively charged nutrients include,  Ca++, Mg++ and K+. You may note that a very important nutrient, Nitrate NO3-, is not listed. This is because it carries a negative charge. Humus has not only negatively charged ions ready for retaining positively charged minerals, some have positively charged ions as well which can hold on to negatively charged minerals such as our precious nitrates and nitrites. However the anion exchange is very low in relation to cation exchange sites and unfortunately doesn’t come into account when talking nitrate availability.

Examples of Cations: NH4+, Ca++, Mg++, K+

Examples of Anions: NO3-,  Cl-, SO4-, PO4-

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How Plants Eat

The surface areas of a plant root hairs contain their own electrical charges. Any time a plant’s root hair penetrates the substrate, it may exchange its own cations for those mounted on humus or clay debris and then absorb the cation nutrient for intake as nourishment. Plant roots use a hydrogen cation (H+) for the exchange. They eject one hydrogen cation for every cation nutrient adsorbed. This keeps a charge balance. This is the way plant life eats. This is a basic function of all plant life.

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If there is a larger concentration of one specific cation over the others in the soil water, that cation will force the other cations off the colloid and the abundant cation element take their place.

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Absorbsion vs Adsorbsion

Positively charged particles are electrostatically attracted to negatively charged particles. We all know this intuitively from our times spent playing with magnets as a child. Whenever positively charged cations affix themselves to negatively charged anions, the cations are “adsorbed” by the anions. Adsorbed is when a substance is affixed to the SURFACE of a particle, in a condensed layer. Absorbsion is when a substance enters INSIDE the body of a particle…  big difference.

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About Anions

Anions, as mentioned earlier, have a negative charge. Some common but important anions are nitrate (NO3-), chloride (Cl-), sulfate (SO4-), and phosphate (PO4-). Unfortunately, soil anions are repelled by way of the negative ion on most substrate particles and therefore stay in solution instead of being adsorbed. Most of these plant nourishment will often be missing from substrate and soils, as they are easily leached out when the plant is watered.This is one more reason bacteria and fungus are so important to us. These microorganisms not only are responsible for breaking down all organic matter, they also supply plants with available anion elements, like nitrate, slowly over time. This is all done without anion exchange from clay or humus. A sufficient percentage of the billion or so bacteria in every square centimeter of soil, die each second to release from their decaying body the nitrates it has accumulated over its life. This to me is an amazing system. Nitrates and anions are also supplied by fungus directly, but dieing bacteria are responsible for the majority of the nutrient availability.